Keeping some money uninvested is useful for dealing with the problems that life regularly throws at you. But keeping more than necessary means losing out on the earnings which this money could be making.
How much emergency fund to keep depends on the sort of emergencies you face. If you are responsible for running a home and a car, you face unexpected repair bills. If you only run a bicycle and you rent, you can expect fewer unexpected bills.

Similarly, if you are in stable employment where you can rely on your wages appearing in your bank each month, you are in a better position to deal with emergencies using a credit card, because you know that you can pay it off from your next pay packet before you begin paying credit card interest.

As a rough guide, many people suggest an initial emergency fund of £1,000, and over time building it to 3-6 months of core spending – i.e. food and bills, but not leisure spending which you could cut out during an emergency such as losing your job.